Organic Substances in Produced and Formation Water from Natural Gas Production in Coal and Shale
Orem, William H.; Tatu, Calin A.; Varonka, Matthew; Pashin, Jack; Engle, Mark
Produced water from unconventional gas extraction in coal and shale is a significant disposal issue because of the large volumes of water involved. Chemical substances in the produced water could pose a possible environmental hazard and human health concern. We examined organic substances in produced water from natural gas producing coals and shales, including: Powder River (freshwater) and Black Warrior Basin (saline) coal and Marcellus shale (hypersaline). Dissolved organic carbon (DOC) concentrations in water from the coals ranged from <1 to 6 mg/L, compared to values up to 3,000 mg/L in water from the Marcellus shale and overlying sandstones. Higher DOC concentrations in the Marcellus could originate from organics in additives used in stimulation fluids.
Extractable organic compounds identified in produced water from the coal included: phenols, biphenyls, heterocyclic compounds, polycyclic aromatic hydrocarbons (PAHs), aliphatic hydrocarbons, and long-chain fatty acids. PAHs, such as naphthalene, phenanthrene, and their alkylated derivatives were the most prevalent compounds identified. Many more unidentified compounds are also present. Concentrations of organics are generally higher in the Black Warrior Basin, which has passed through the oil window, than in the Powder River Basin, where coal is thermally immature. The presence of long-chain fatty acids and low acetate concentrations may further reflect biogenic production of methane along a methyl fermentation pathway within the low-rank coal beds of the Powder River Basin. In the Black Warrior Basin, biogenesis along a CO2 reduction pathway, and the long-chain fatty acids coupled with the lack of acetate may reflect elevated thermal maturity. Some identified compounds are toxic, but are unlikely to have acute health effects at the observed concentrations. Chronic health and environmental effects from long periods of low-level exposure are unknown. In the Marcellus shale and overlying sandstone extractable hydrocarbons in produced water were similar, and included alkanes, biocides, ethylene glycol, and heterocyclic compounds. The alkanes, many of the heterocyclics, and other identified organics (biocides, ethylene glycol) could originate from chemicals added during hydrofracturing operations. Produced water from the shale contains far higher concentrations of potentially toxic organics than produced water from the coal, and care should be exercised in the disposal of these fluids.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013